Process for production of carbon fibers

ABSTRACT

A PROCESS FOR THE MANUFACTURE OF SUPERIOR CARBON OR GRAPHITE FIBERS FROM POLYMETHYL VINYL KETONE, AS RAW MATERIAL, WHICH COMPRISES THE STEPS OF MELT-SPINNING FIBERS THEREFROM, STRETCH-ORIENTING THE FIBERS IN THE AXIAL DIRECTION THEREOF CYCLIZING THE COMPOUND MATERIAL CONSTITUTING THE FIBERS BY DEHYDRATION, AND CARBONIZING THE CYCLIZED FIBERS. THE CARBON FIBERS THUS OBTAINED ARE READILY CONVERTIBLE INTO GRAPHITE FIBERS BY A SIMPLE ADDITIONAL PROCESS OF GRAPHITIZATION. DESIRABLY, THE CYCLIZED POLYMETHY VINYL KETONE FIBERS ARE THEN DENATURED AS TO HAVE SUFFICIENT HEAT RESISTIVITY TO UNDERGO THE ENSUING STEP OF CARBONIZATION WITHOUT SUFFERING EIGHER DEGASIFICATION OR DEFORMATION.

United States Patent O" 3,666,417 PROCESS FOR PRODUCTION OF CARBON FIBERS Tadashi Araki, Hitoshi Takita, and Kiro Asano, Tokyo, Japan, assignors to Kureha Kagaku Kogyo Kabushiki Kaisha, To o-to, Japan No Drawiiig: Filed May 15, 1970, Ser. No. 37,865 Claims priority, application Japan, May 17, 1969, 44/37,725 Int. Cl. C01b 31/07 US. Cl. 23-2091 5 Claims ABSTRACT OF THE DISCLOSURE A process for the manufacture of superior carbon or graphite fibers from polymethyl vinyl ketone, as raw material, which comprises the steps of melt-spinning fibers therefrom, stretch-orienting the fibers in the axial direction thereof cyclizing the compound material constituting the fibers by dehydration, and carbonizing the cyclized fibers. The carbon fibers thus obtained are readily convertible into graphite fibers by a simple additional process of graphitization. Desirably, the cyclized polymethyl vinyl ketone fibers are then denatured as to have sufficient heat resistivity to undergo the ensuing step of carbonization without suffering either degasification or deformation.

BACKGROUND OF THE INVENTION This invention relates to a process for the manufacture of carbon fibers. More specifically, it relates to a process for the manufacture of carbon fibers from a novel material of polymethyl vinyl ketone.

Heretofore, carbon fibers have been produced by the carbonization of various organic fibers, including those high polymers such as polyacrylonitrile, polybutadiene as well as rayon fibers. Yet, the list of the materials considered employable for the manufacture of carbon fibers has long been a pretty well established one. The present invention, however, is based on discovery that polymethyl vinyl ketone, which has so far been altogether disregarded as a material for the production of carbon fibers, is not only capable of producing carbon fibers, but also capable of obtaining under certain conditions remarkably excellent carbonaceous or graphite fibers.

The carbon fibers obtained by the carbonization of polymethyl vinyl ketone fibers has very high uniformity in both their properties and shape, because the polymethyl vinyl ketone fibers are so denatured as to have heat resistivity sufiicient to undergo the carbonization treatment without causing undue degasification or change in shape. Here, the term carbonization treatment is meant, for the sake of convenience, by the one conducted at temperatures not exceeding 2,000 C.

Polymethyl vinyl ketone can be readily cyclized by causing it to dehydrate by contact reaction with ammonia, alkali, acid, or appropriate organic solvents such as, for example, solutions of alcohol and tetrahydrofuran, with or without the use of heat and/or ultraviolet rays to accelerate the reaction, or, most simply, by causing it to dehydrate by means of heating. The contact reaction with the reagents can be smoothly performed by using them in a gaseous state in the case of ammonia, and a liquid state in the case of alkali or acid. These dehydration re- 3,666,417. Patented May 30, 1972 actions can be represented by the following structural formulas.

The polymethyl vinyl ketone fibers thus dehydrated and cyclized, or modified polymethyl vinyl ketone fibers obtained by contacting the polymethyl vinyl ketone fibers with one or more kinds of oxidizing gas such as, for example, ozone, oxygen, air, oxides of nitrogen, S0 and halogen gas at a temperature of less than 350 C. to be oxidized and dehydrogenated for sufiicient heat resistance have been found to be converted to carbon fibers with the least degasification and without causing change in shape whatsoever.

This treatment can be done simultaneously with the above-mentioned cyclization by dehydration treatment. In other words, by heating the fibers in the presence of the oxidizing gas, it is possible to attain at the same time both purposes of cyclization and oxidation. Also, when gaseous ammonia is used with one or more kinds of oxidizing gas which does not react directly such as, for example, oxygen, ozone, and air, carbon fibers of higher carbon yield can be more readily obtained. (The carbon yield can be represented by weight percent of residual carbon atom in the final product.)

The new efiect brought about by ammonia has yet to be clarified. It has however been found out by measurements such as elementary analysis and infrared ray spectrum, etc. that nitrogen atom introduced into the treated fibers does contribute to acceleration of crosslinking and increase in the softening point of the substance to be treated.

It has also been discovered by the present inventors that the higher the isotacticity of polymethyl vinyl ketone is, and the higher the degree of crystal orientation of the fibers is, the more remarkably does the compound cyclize, which contributes greatly to the improvement in carbonization yield and elasticity of the final products. Hence, polymethyl vinyl ketone, with its intrinsic properties discussed above, has proved to be a promising material suitable for the production of carbon fibers.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a method for the manufacture of superior carbon fibers from polymethyl vinyl ketone which has hitherto been unrecognized as suitable material.

It is another object of the invention to provide a method for the manufacture of superior carbon fibers from polymethyl vinyl ketone by cyclizing the fiber structure prior to their carbonization so as to minimize degassification and, moreover, cause no change whatsoever in shape when carbonized successively to be turned into the desired carbon fibers.

These and various other objects of the present invention as well as the characteristic features thereof will become more apparent and understandable from the following detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION Polymethyl vinyl ketone fibers to be converted into the desired carbon fibers are obtainable by the processes of spinning and stretch-orientation as is the case with those fibers produced from the ordinary high polymers. The abovementioned cyclization treatment may generally be given to the polymethyl vinyl ketone material at any stage of the above two processes, although, in order to secure smooth progress in the overall processes, it is preferable to do this at the time of stretch-orientation, when the compound material is already given a required shape. It has, however, been discovered that the cyclization treatment in the main should be preferably given to the fibers simultaneously with the stretch-orientation for the purpose of improving the modulous of elasticity of carbonaceous or graphite fibers.

In particular, polymethyl vinyl ketone material, the melting point of which increased due to the cyclization and the cross-linking among the molecules taken place prior to the spinning process and which therefore becomes inadequate for the melt-forming, may be dry-spun, or wet-spun by use of an arbitrarily selected solvent such as formic acid, tetrahydrofuran and pyridine. Such cyclization of the polymethyl vinyl ketone material may, in some case, be done with a view to carrying out the carbonization treatment after shaping in a quicker and more complete manner as well as obtaining the fibers of varied cross sections. The cyclization of polymethyl vinyl ketone material can be easily taken place by the use of various reagents such as phosphoric acid, sulfuric acid, chloric acid, nitric acid, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, or alkaline earth metals. The kinds of phosphoric acid are orthophosphoric acid, meta-phosphoric acid, pyro-phosphoric, poly-phosphoric acid.

The modified polymethyl vinyl ketone fibers, thus cyclized and made heat-resistive, are converted into carbon fibers by subjecting them to carbonization treatment carried out in non-reactive atmosphere. If desired, the carbonized fibers may be further converted into very superior graphite fibers by subjecting them to graphitization treatment in an inactive atmosphere at a temperature of more than 2,000 C.

This superiority is considered due to the graphite structure of the fibers thus obtained, which consists of not only the main carbon chain in polymethyl vinyl ketone oriented in the axial direction of the fiber by the stretch-orientation process, etc., but also the second main carbon chain composed of methyl and carbonyl groups, and formed by the cyclization, these chains being sufficiently long, and neither broken nor damaged by the graphitization treatment. Thus, the superiority of polymethyl vinyl ketone as a material for the manufacture of carbon or graphite fibers can be deduced by that the molecules of the compound material form a pair of elongated main carbon chains or a laddered ring srtucture and that these carbon structures are capable of remaining in the carbon fiber structure as the matrix for the graphite crystals. The abovementioned point is considered to be the essential difference of the carbon fibers of the present invention made of the polymethyl vinyl ketone material from the conventional high polymer carbon fibers mentioned above. Consequently, the most pronounced features of the present invention lies in the use of such material for the carbon fiber manufacture.

The fibers having such pair of main carbon chains as matrix for the graphite crystals may improve elasticity thereof by subjecting them to a stretch-out orientation in the course of the graphitization treatment. However, as the graphite crystals in the fibers are primarily and sufficiently oriented in the axial direction thereof without stretching them, the elasticity of the graphite fibers to be obtained can be sufficiently high.

The invention will now be described in reference to its preferred examples, but it is not to be considered a limitation to the scope of the present invention.

EXAMPLE I Isotactic polymethyl vinyl ketone material was meltspun into fibers at a temperature of 195 C. (383 F.), after which the fibers were subjected to dehydration and cyclization treatments for 1 hour while they were being stretch-oriented in a 15% aqueous solution of polyphosphoric acid at a temperature of C. (167 F.), whereby dark brown cyclized polymethyl vinyl ketone fibers of approximately 12 microns in diameter were obtained. The fibers did not indicate any definite melting point, though it could recognized by the elementary analysis, infrared ray analysis and other means that sufficient cyclization took place in the structure.

After sufficient drying, when the above denatured polymethyl vinyl ketone fibers were heat-treated in a nitrogen gas at a temperature range of from room temperature up to l,400 C. (2,252 F.) at a rise rate of 3 C. (37.4 F.) per minute, they turned into highly lustrous carbon fibers. The carbon yield from the original polymethyl vinyl ketone to carbon fibers was as high as 81 percent.

The carbon fibers were then heat-treated in an argon gas at temperatures elevated up to 2,000 C. (3,650 F.) at a rise rate of 50 C. (122 F.) per minute, thereafter they were further treated at temperatures up to 2,800 C. (5,072 F.) for 15 minutes inclusive of retention time at this temperature, whereby graphite fibers were obtained. The fibers showed an average diameter of 8.2 microns, tensile strength of 16 t./cm. and modulus of elasticity of 2,300 t./cm. Through X-ray observations, the orientation of the graphite crystals in the axial direction of the fibers was clearly recognized.

EXAMPLE II Fibers were melt-spun at a temperature of C. (228 F.) from polymethyl vinyl ketone which Was obtained by radical polymerization due to heat, has less isotacticity, and is presumed to have caused a partial thermal cyclization in the structure. The fibers were stretch-oriented in the axial direction thereof in a bath of warm water heated up to 40 C. (105 F.), whereby light yellow fibers of approximately 10 microns in diameter were obtained.

As a result of treating the fiber in concentrated sulfuric acid at room temperature for 20 minutes for dehydration and cyclization, they turned into deep brown fibers. The fibers were sufficiently rinsed, after which they were treated for oxidation and dehydrogenation in air containing 5% by volume of N0 at a temperature of C. (303 F.), whereby black modified polymethyl vinyl ketone fibers were obtained. These fibers were carbonized in nitrogen gas initially at temperatures of from 300 C. (573 F.) to 650 C. (1.203 F.) at a rise rate of 2 C. (36.6 F.) per minute and then from 650 C. to 1,000 C. (1,833 F.) at a rise rate of 10 C. (51 F.) per minute. The carbon fibers thus obtained were flexible in touch, had an average diameter of 7.2 microns, and indicated tensile strength of 9 t./cm. The carbon yield from the original polymethyl vinyl ketone to carbon fiber was as high as 72 percent.

EXAMPLE III The melt-spun and stretch-oriented fibers in Example II were contacted with an atmosphere of 80% ammonia and 20% oxygen at a temperature of from room-temperature to 220 C. at a temperature rise rate of 1 C. per minute. It was found out by the elementary analysis as well as the infrared ray spectrum that 6.05% nitrogen and 12.73% oxygen were introduced into the fibers. After this cyclization treatment, orange colored fibers were obtained.

Then the fibers were subjected to carbonization in nitrogen gas to an elevated temperature of 1,000 C. at a temperature rise rate of 1 C. per minute. The carbon yield at that time was 90%. The carbon fibers thus obtained had an average diameter of 15 microns, tensile strength of 8 tons/cmF, and elongation of 2% EXAMPLE IV The melt-spun and stretch-oriented fibers in Example I were contacted with 100% NH at a temperature rang ing from room temperature to 200 C. at a rise rate of 1 C./min. This cyclizationtreated fibers were contacted with air containing 1% by volume of N at a temperature of 110 C. for ten minutes. It was found out by the elementary analysis as well as the infrared ray spectrum that 7.03% nitrogen and 13.29% oxygen were introduced into the fibers. The fibers were then carbonized in a nitrogen gas up to 1,500 C. at a rise rate of C./min., the carbon yield at that time having been 98%. The carbon fibers thus obtained had an average diameter of microns, tensile strength of 8.5 tons/cmF, and elongation of 2%. The carbon fibers thus obtained were further graphitized by heating in argon gas at a temperature of 2,800 C. for 10 minutes. The graphite fibers had 18 tons/ cm. in tensile strength and 2,500 ton/cm. in modulus of elasticity.

We claim:

1. In a process for manufacturing carbon fibers from polymethyl vinyl ketone as raw material through the process steps of melt-spinning the raw material into fine fibrous form, oxidizing the fibers in an oxidizing atmosphere at a temperature up to 350 C. and carbonizing the thus treated fibers, the improvement which comprises stretchorienting said spun fibers in the axial direction thereof; and subjecting the stretched fibers to a dehydration and cyclization treatment in a reagent selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, ammonia in solution and gaseous ammonia.

2. The process as claimed in claim 1, wherein said cyclization treatment is carried out simultaneously by heating in the presence of a gaseous mixture containing ammonia and oxygen at a temperature up to 350 C., while said polymethyl vinyl ketone fibers are being oxidized and dehydrogenated.

3. The process as claimed in claim 1, wherein said cyclization treatment is carried out simultaneously while said polymethyl vinyl ketone fibers are being stretchoriented.

4. The improvement according to claim 1, in which said cyclization by dehydration is carried out in gaseous ammonia.

5. The improvement according to claim 1, in which said cyclization by dehydration is carried out in an atmosphere containing gaseous ammonia.

References Cited UNITED STATES PATENTS 3,006,894 10/1961 Evans et al. 26063 3,392,216 7/1968 Otani 23209.1 X 3,412,062 11/1968 Johnson et a1. 26037 3,497,318 2/1970 Noss 23-209.1 3,539,295 11/1970 Bam 23209.1

FOREIGN PATENTS 1,487,322 5/1967 France 23-2091 EDWARD J. MEROS, Primary Examiner US. Cl. X.R. 

